Government forces launched a counter-attack to recapture a gas field seized by Islamic extremists in Syria, activists said yesterday, as the death toll from three days of fighting rose to more than 270.

The intense fighting in the Shaer field, in the desert region of Palmyra in the central province of Homs, has been among the deadliest between government forces and the Islamic State of Iraq and the Levant (Isis) since the start of the Syrian civil war more than three years ago. Isis has seized a huge chunk of territory straddling the Iraq-Syria border, where it has declared a caliphate, or Islamic state. Its fighters have also captured much of Syria's oil-rich eastern province of Deir el-Zour.

The Syrian Observatory for Human Rights said Syrian special forces attacked the Shaer field late on Friday, regaining parts of it. Homs-based activist Beibares Tellawi described the battles as "hit-and-run attacks".

The observatory said that the number of troops, guards and workers killed in the gas field since it was captured on Thursday had risen to 270, adding that some were captured and killed by militants. The fate of 90 gas-field workers and guards is unknown.

According to Syria's state news agency, Sana, the army has also "hit terrorist dens", killing and wounding scores of militants in villages and towns in Daraa.

Meanwhile, in the rebel-held Damascus suburb of Douma, a car bomb killed at least nine people yesterday, the observatory said.

In Iraq it was reported that Isis had issued an ultimatum to the dwindling Christian population to convert to Islam, pay a religious levy (jizya), or face death. That came as a series of bombs killed at least 27 people across Baghdad.